Sir Simon Milton, who has died suddenly aged 49, was the chief of staff to Boris Johnson and one of the mayor of London's deputies. He had been a leading politician within the capital's government for almost 25 years, having led Westminster city council prior to taking charge of the Johnson administration when it was in trouble during the late summer of 2008. He was a clever and unassuming man, a self-professed "one-nation" Tory with a highly effective style of government.

By moving from local government to City Hall, Milton made what must be seen as a big shift within the complex world of London government. The relationship between boroughs and the mayor can be fractious. But, having run a borough for eight years, Milton had the sensitivity and experience to maintain a good relationship with council leaders when he was put in charge of city-wide planning. As Westminster leader, he had worked with Ken Livingstone during the latter's period as mayor on projects as varied as regenerating Oxford Street and an effort to reduce the number of pigeons in Trafalgar Square.

Milton grew up in Cricklewood, north-west London, the son of Clive and Ruth Milton, who established the Sharaton chain of London patisseries, having started with a single baker's shop in Cricklewood Broadway. Milton was educated at St Paul's school, Hammersmith, and read history at Gonville and Caius College, Cambridge, where he was chairman of the university's Conservative Association and president of the Cambridge Union. He took a master's degree in professional studies at Cornell University, New York.

He was elected to Westminster city council in 1988, serving on a number of committees before becoming leader in 2000. He maintained a private sector career within a public relations firm during his early years on the council. As leader, he maintained Westminster's high profile, but moved it on from the Thatcherite image developed by Dame Shirley Porter and her immediate successors. Despite its glittering southern part, much of the north of the borough was, and remains, among the most deprived places in Britain. Milton's moderate administration recognised this reality.

Well before becoming leader, in 1990 he had been diagnosed with leukaemia. In 1998 he underwent a bone-marrow transplant, using cells provided by his sister, Lisa. As a result of this procedure, his immune system was weakened, leading to a bout of pneumonia which seriously damaged his lungs. His health had been a problem ever since.

Milton was knighted for services to local government in 2006. Prominence at Westminster council led to his election to chair the Local Government Association (LGA) in 2007, thus representing all councils on the national political stage. The significance of this role led to the expectation he might become a minister (in the House of Lords) if the Conservatives were to win the coming general election.

In the event, just a year after becoming head of the LGA, he moved on to become Johnson's deputy mayor for policy and planning and, latterly, his chief of staff. He arrived at City Hall at a turbulent time in the summer of 2008, early in the Johnson administration, after one of his earlier-appointed deputy mayors had had to resign.

Subsequently, Milton was able to get a grip on City Hall and impose normality. His experiences within local government had prepared him well. His key role during the past three years was to oversee the process of revising the London Plan, the mayor's key document in guiding the future of the city. The outer boroughs of London were given new priority in the process.

Despite continuing problems with his health, he was able to provide the Johnson regime with stability until his untimely death. He was an intelligent, calm and modest man whose influence on London government will be felt for years to come.

He is survived by his civil partner, Robert Davis, his mother and his sister.

• Simon Henry Milton, politician, born 2 October 1961; died 11 April 2011